This Application is a Continuation-in-Part of copending U.S. patent application Ser. No. 09/437,346, Entitled xe2x80x9cDUAL-LAYER METAL FOR FLAT PANEL DISPLAYxe2x80x9d to Chakravorty et al. filed Nov. 9, 1999. This Application is also related to United States Patent Application entitled xe2x80x9cMULTILAYER ELECTRODE STRUCTURE AND METHOD FOR FORMING MULTILAYER ELECTRODE STRUCTURE FOR A FLAT PANEL DISPLAY DEVICExe2x80x9d, which is filed concurrent with the filing of the present Application.
The present claimed invention relates to the field of flat panel displays. More specifically, the present claimed invention relates to a flat panel display and methods for forming a flat panel display having emitter electrode metal which provides good conductivity and which resists damage in subsequent process steps.
A Cathode Ray Tube (CRT) display generally provides the best brightness, highest contrast, best color quality and largest viewing angle of prior art computer displays. CRT displays typically use a layer of phosphor which is deposited on a thin glass faceplate. These CRTs generate a picture by using one to three electron beams which generate high energy electrons that are scanned across the phosphor in a raster pattern. The phosphor converts the electron energy into visible light so as to form the desired picture. However, prior art CRT displays are large and bulky due to the large vacuum envelopes that enclose the cathode and extend from the cathode to the faceplate of the display. Therefore, typically, other types of display technologies such as active matrix liquid crystal display, plasma display and electroluminescent display technologies have been used in the past to form flat panel displays.
Recently, a thin flat panel display commonly referred to as a field emission display (FED) has been developed which uses the same process for generating pictures as is used in CRT devices. These FEDs use a backplate including a matrix structure of rows and columns of electrodes. One such FED flat panel display is described in U.S. Pat. No. 5,541,473 which is incorporated herein by reference. Typically, the backplate is formed by depositing a cathode structure (electron emitting) on a glass plate. The cathode structure includes emitters that generate electrons. The backplate typically has an active area surface within which the cathode structure is deposited. Typically, the active area surface does not cover the entire surface of the glass plate and a thin strip is left around the edges of the glass plate. The thin strip is referred to as a border or a border region. Conductive traces extend through the border to allow for electrical connectivity to the active area surface.
Prior art flat panel displays include a thin glass faceplate (anode) having a layer of phosphor deposited over the surface of the faceplate. A conductive layer is deposited on the glass or on the phosphor. The faceplate is typically separated from the backplate by about 1 millimeter. The faceplate includes an active area surface within which the layer of phosphor is deposited. The faceplate also includes a border region. The border is a thin strip that extends from the active area surface to the edges of the glass plate. The faceplate is attached to the backplate using a glass sealing structure. This sealing structure is typically formed by melting a glass frit in a high temperature heating step. This forms an enclosure which is pumped out so as to produce a vacuum between the active area surface of the backplate and the active area surface of the faceplate.
Prior art cathodic structures are typically formed by depositing a first layer of metal over a glass plate (first metal layer). This first metal layer is then masked and etched so as to form emitter electrodes (rows or columns). Typically, a resistive layer formed of silicon carbide (SiC), Cermet, or a combination of SiC and Cermet is deposited over the emitter electrode metal. A dielectric layer is then deposited. A second layer of metal is then deposited over the surface of the cathodic structure. A series of mask and etch steps are then performed so as to form gate electrodes (rows or columns). The mask and etch steps also form openings in the gate electrode metal which extend through the dielectric layer so as to expose portions of the resistive layer. Emitters are formed over the exposed portions of the emitter electrode metal and within the openings in the gate metal by a series of deposition and etch steps. Individual regions of the cathode are selectively activated by applying electrical current to selected conductive strips of emitter electrode metal and selected conductive strips of gate metal so as to generate electrons which strike the phosphor so as to generate a display within the active area surface of the faceplate. These FEDs have all of the advantages of conventional CRTs but have the great advantage of being much thinner.
The first metal layer of a FED is typically formed of an alloy of nickel (approx. 92%) and vanadium (approx. 8%). A nickel vanadium alloy is used since it gives a good electrical bond with the overlying resistive layer and because it is resistant to damage and contamination in subsequent process steps. However, the resistivity of the nickel vanadium layer is approximately 55 micro-ohms-centimeter. This high resistivity causes signal delay. Signal delay causes decreased performance and inconsistent display quality. In addition, nickel vanadium alloy is expensive.
In an attempt to overcome the problems associated with the use of nickel vanadium alloy in emitter electrode metal formation, manufacturers have attempted to use less resistive materials such as aluminum. However, many of these less resistive materials unfortunately do not meet process compatibility requirements. In addition, many of these less resistive materials typically do not form a sufficient electrical contact with the overlying resistive layer to function effectively. This is primarily due to the native oxide that forms on the surface of the conductive layer inhibiting current flow. In addition, subsequent process steps damage and contaminate the surface of the aluminum. In particular, the alkaline and acidic solutions used in subsequent process steps attack aluminum. Moreover, subsequent rinsing and cleaning steps may leave deposits that adhere to the surface of the aluminum. These contaminants further degrade the quality of electrical contact between the emitter electrode metal and the resistor.
One of the reasons that aluminum forms a poor electrical bond with the overlying resistive layer is oxidation of the surface of the aluminum. This oxidation results from exposure to atmospheric conditions. Prior art methods have attempted to get a good electrical bond between the Aluminum and the overlying resistive layer by performing an etch on the aluminum layer such as a sputter etch. This sputter etch removes accumulated oxidation (aluminum oxide). Though sputter etching gives good results for small surface areas, sputter etching does not give consistent coverage across the large surface areas required for current FEDs. For the above reasons, aluminum has significant disadvantages when used in forming emitter electrode metal in prior art FED devices.
Accordingly, what is needed is a FED with emitter electrode metal which minimizes signal delay and which meets signal propagation and other performance criteria and process compatibility criteria. In addition, a FED is needed that has emitter electrode metal which is easy to deposit and etch and which can be formed using current processing techniques. Moreover, processing methods for forming a FED with emitter electrode metal that has low resistivity and that forms a good bond with a resistive layer are required. Furthermore, processing methods are needed for forming a FED with emitter electrode metal that is resistant to damage during subsequent processing steps. The present invention meets the above needs.
The present invention provides a field emission display (FED) which includes an improved cathodic structure. The cathodic structure includes emitter electrode metal which is highly conductive. The emitter electrode metal is formed using aluminum which is overlain by a thin cladding layer.
In one embodiment of the present invention, a faceplate is formed by depositing luminescent material within an active area surface formed on a glass plate. A cathodic structure is formed within an active area on a backplate. Walls are attached to either the faceplate or the backplate. A glass sealing material is placed within the border of the faceplate. The backplate is then placed over the faceplate such that the walls and the glass frit are disposed between the faceplate and the backplate. The assembly is then sealed by thermal processing and evacuation steps so as to form a complete FED.
The cathodic structure includes rows of metal strips aligned roughly parallel to each other (herein referred to as xe2x80x9cemitter electrodesxe2x80x9d). Each strip includes a layer of aluminum overlain by a layer of cladding material. A resistive layer overlies the emitter electrode metal. A dielectric layer overlies the resistive layer. Gate metal overlies the dielectric layer. Gate metal are rows of strips of conductive material which are aligned roughly parallel to each other. Openings which extend through the gate metal and through the dielectric layer expose portions of the resistive layer. Emitters are formed within the openings in the gate metal and the dielectric layer such that they are electrically coupled to the resistive layer. In operation, electrical current is applied to one or more strips of the emitter electrode metal and to one or more strips of gate metal such that emitters disposed over the strips of emitter electrode metal to which current is applied and within openings in the strips of gate metal to which current is applied are engaged such that they emit electrons. These electrons strike the phosphor deposited on the faceplate so as to produce a visible display.
The use of aluminum and cladding material to form emitter electrode metal gives emitter electrode metal segments which are highly conductive due to the high conductivity of aluminum. By using processing steps and a cladding material which will not interdiffuse in subsequent thermal process steps, emitter electrode metal is formed which maintains good electrical conductivity with overlying structures even after high temperature process steps. A cladding material which forms a good bond with the overlying resistive layer is used. In one embodiment, a refractory metal such as tantalum is used as a cladding material. When using silicon carbide to form the resistive layer a bond which has good electrical conductivity is formed between the tantalum layer and the silicon carbide. Thus, the resulting structure has very high electrical conductivity (through the aluminum layer) and high conductivity into the resistive layer.
In one embodiment, aluminum is deposited, masked and etched to form aluminum strips. A cladding layer of tantalum is then deposited over the aluminum strips. An etch is then performed so as to remove some or all of the tantalum between adjacent strips of aluminum and tantalum.
In an alternate embodiment, the aluminum and the cladding layer are deposited sequentially in a vacuum deposition chamber. The resulting structure is then masked and etched to form strips having aluminum overlain by the cladding layer. The sequential deposition process gives a more uniform cladding layer since oxidation between the aluminum layer and the cladding layer is avoided and since contamination that may occur from masking, etching, and photoresist removal steps is avoided.
The present invention produces a structure which has favorable conductivity characteristics and which has conductivity characteristics which are consistent throughout the emitter electrode metal. In addition, as a result of the cladding layer""s resistance to damage, the emitter electrode metal is not damaged in process steps subsequent to the step of depositing the cladding layer.
The favorable conductivity characteristics are consistent throughout the emitter electrode metal as a result of the cladding layer""s resistance to damage in subsequent process steps. In particular, tantalum and other refractory metals resists damage when exposed to etchant chemicals and processing chemicals such as alkaline and acidic solutions which are commonly used in subsequent process steps. Aluminum is desirable as a conductor since it is commonly used in electronic circuit devices and because it is inexpensive and it has good conductivity.
In another embodiment of the present invention, two-layer electrode structures are disclosed that include chromium-containing material. One two-layer electrode structure includes a layer of chromium, and a layer of nickel and vanadium alloy. Three-layer structures are also disclosed. In yet another embodiment of the present invention, one or more resistor layer is used to prevent damage to an electrode.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art in light of the following detailed description of the preferred embodiments that are illustrated in the various drawing figures.